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    SubjectRe: [PATCH] SPARC64: fix iommu sg chaining
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    From: Jens Axboe <jens.axboe@oracle.com>
    Date: Wed, 17 Oct 2007 11:16:29 +0200

    > On Wed, Oct 17 2007, David Miller wrote:
    > > From: Jens Axboe <jens.axboe@oracle.com>
    > > Date: Wed, 17 Oct 2007 10:45:28 +0200
    > >
    > > > Righto, it's invalid to call sg_next() on the last entry!
    > >
    > > Unfortunately, that's what the sparc64 code wanted to do, this
    > > transformation in the sparc64 sg chaining patch is not equilavent:
    > >
    > > - struct scatterlist *sg_end = sg + nelems;
    > > + struct scatterlist *sg_end = sg_last(sg, nelems);
    > > ...
    > > - while (sg < sg_end &&
    > > + while (sg != sg_end &&
    >
    > Auch indeed. That'd probably be better as a
    >
    > do {
    > ...
    > } while (sg != sg_end);

    Ok, next bug, introduced by this change:

    commit f565913ef8a8d0cfa46a1faaf8340cc357a46f3a
    Author: Jens Axboe <jens.axboe@oracle.com>
    Date: Fri Sep 21 10:44:19 2007 +0200

    block: convert to using sg helpers

    Convert the main rq mapper (blk_rq_map_sg()) to the sg helper setup.

    Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

    Specifically this part:

    new_segment:
    - memset(&sg[nsegs],0,sizeof(struct scatterlist));
    - sg[nsegs].page = bvec->bv_page;
    - sg[nsegs].length = nbytes;
    - sg[nsegs].offset = bvec->bv_offset;
    + sg = next_sg;
    + next_sg = sg_next(sg);

    + sg->page = bvec->bv_page;
    + sg->length = nbytes;
    + sg->offset = bvec->bv_offset;

    You can't remove that memset(), it's there for a reason. The IOMMU
    layers depended upon the code zero'ing out the whole scatterlist
    struct, there might be more to it than page, length and offset :-)

    In sparc64's case, this zero'd the dma_address and dma_length members
    and the mapping algorithms use that to their advantage.
    -
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